16,243 research outputs found
Locality with staggered fermions
We address the locality problem arising in simulations, which take the square
root of the staggered fermion determinant as a Boltzmann weight to reduce the
number of dynamical quark tastes. A definition of such a theory necessitates an
underlying local fermion operator with the same determinant and the
corresponding Green's functions to establish causality and unitarity. We
illustrate this point by studying analytically and numerically the square root
of the staggered fermion operator. Although it has the correct weight, this
operator is non-local in the continuum limit. Our work serves as a warning that
fundamental properties of field theories might be violated when employing
blindly the square root trick. The question, whether a local operator
reproducing the square root of the staggered fermion determinant exists, is
left open.Comment: 24 pages, 7 figures, few remarks added for clarity, accepted for
publication in Nucl. Phys.
Phase transition of the nucleon-antinucleon plasma at different ratios
We investigate phase transitions for the Walecka model at very high
temperatures. As is well known, depending on the parametrization of this model
and for the particular case of a zero chemical potential (), a first
order phase transition is possible \cite{theis}. We investigate this model for
the case in which . It turns out that, in this situation, phases
with different values of antinucleon-nucleon ratios and net baryon densities
may coexist. We present the temperature versus antinucleon-nucleon ratio as
well as the temperature versus the net baryon density for the coexistence
region. The temperature versus chemical potential phase diagram is also
presented.Comment: 5 pages, 8 figure
Magnetic structure of the edge-sharing copper oxide chain compound NaCu2O2
Single-crystal neutron diffraction has been used to determine the
incommensurate magnetic structure of NaCu2O2, a compound built up of chains of
edge-sharing CuO4 plaquettes. Magnetic structures compatible with the lattice
symmetry were identified by a group-theoretical analysis, and their magnetic
structure factors were compared to the experimentally observed Bragg
intensities. In conjunction with other experimental data, this analysis yields
an elliptical helix structure in which both the helicity and the polarization
plane alternate among copper-oxide chains. This magnetic ground state is
discussed in the context of the recently reported multiferroic properties of
other copper-oxide chain compounds
Incommensurate spin density modulation in a copper-oxide chain compound with commensurate charge order
Neutron diffraction has been used to determine the magnetic structure of
NaCuO, a stoichiometric compound containing chains based on
edge-sharing CuO plaquettes. The chains are doped with 2/5 hole per Cu site
and exhibit long-range commensurate charge order with an onset well above room
temperature. Below K, the neutron data indicate long-range collinear
magnetic order with a spin density modulation whose propagation vector is
commensurate along and incommensurate perpendicular to the chains. Competing
interchain exchange interactions are discussed as a possible origin of the
incommensurate magnetic order
Phonon-assisted tunneling in the quantum regime of Mn12-ac
Longitudinal or transverse magnetic fields applied on a crystal of Mn12-ac
allows to observe independent tunnel transitions between m=-S+p and m=S-n-p
(n=6-10, p=0-2 in longitudinal field and n=p=0 in transverse field). We observe
a smooth transition (in longitudinal) from coherent ground-state to thermally
activated tunneling. Furthermore two ground-state relaxation regimes showing a
crossover between quantum spin relaxation far from equilibrium and near
equilibrium, when the environment destroys multimolecule correlations. Finally,
we stress that the complete Hamiltonian of Mn12 should contain odd spin
operators of low order
Looming struggles over technology for border control
New technologies under development, capable of inflicting pain on masses of people, could be used for border control against asylum seekers. Implementation might be rationalized by the threat of mass migration due to climate change, nuclear disaster or exaggerated fears of refugees created by governments. We focus on taser anti-personnel mines, suggesting both technological countermeasures and ways of making the use of such technology politically counterproductive. We also outline several other types of ‘non-lethal’ technology that could be used for border control and raise human rights concerns: high-powered microwaves, armed robots, wireless tasers, acoustic devices/vortex rings, ionizing and pulsed energy lasers, chemical calmatives, convulsants, bioregulators and malodurants. Whether all these possible border technologies will be implemented is a matter for speculation, but their serious human rights implications warrant advance scrutiny
Magnetic properties of PdAs2O6: a dilute spin system with an unusually high N\'eel temperature
The crystal structure and magnetic ordering pattern of PdAs2O6 were
investigated by neutron powder diffraction. While the magnetic structure of
PdAs2O6 is identical to the one of its isostructural 3d-homologue NiAs2O6, its
N\'{e}el temperature (140 K) is much higher than the one of NiAs2O6 (30 K).
This is surprising in view of the long distance and indirect exchange path
between the magnetic Pd ions. Density functional calculations yield
insight into the electronic structure and the geometry of the exchange-bond
network of both PdAs2O6 and NiAs2O6, and provide a semi-quantitative
explanation of the large amplitude difference between their primary exchange
interaction parameters
Hysteresis in the de Haas-van Alphen Effect
A hysteresis loop is observed for the first time in the de Haas-van Alphen
(dHvA) effect of beryllium at low temperatures and quantizing magnetic field
applied parallel to the hexagonal axis of the single crystal. The irreversible
behavior of the magnetization occurs at the paramagnetic part of the dHvA
period in conditions of Condon domain formation arising by strong enough dHvA
amplitude. The resulting extremely nonlinear response to a very small
modulation field offers the possibility to find in a simple way the Condon
domain phase diagram. From a harmonic analysis, the shape and size of the
hysteresis loop is constructed.Comment: 4 pages, 5 figures, submitted to PR
InAs-AlSb quantum wells in tilted magnetic fields
InAs-AlSb quantum wells are investigated by transport experiments in magnetic
fields tilted with respect to the sample normal. Using the coincidence method
we find for magnetic fields up to 28 T that the spin splitting can be as large
as 5 times the Landau splitting. We find a value of the g-factor of about 13.
For small even-integer filling factors the corresponding minima in the
Shubnikov-de Haas oscillations cannot be tuned into maxima for arbitrary tilt
angles. This indicates the anti-crossing of neighboring Landau and spin levels.
Furthermore we find for particular tilt angles a crossover from even-integer
dominated Shubnikov-de Haas minima to odd-integer minima as a function of
magnetic field
- …